The present invention relates to internal-combustion engines. More particularly, this invention relates to balancing means for internal-combustion engines capable of balancing the principal types of dynamic unbalance created in such engines.
The principal application considered is for engines used to power automobiles. Automotive engines have several important requirements to meet in addition to that of producing power. For example, they must be compact in order to leave as much space as possible for the passenger compartment and for important accessory equipment. In addition, automotive engines must operate at high speed with a minimum of vibration. In order to accomplish this, the engine must have a high degree of balance in its moving parts.
The balance of automotive engines of the piston or reciprocating type is complicated because of the reciprocating motion of the piston. This motion has two important harmonics. The primary harmonic results in a shaking force which varies through a cycle having a frequency equal to the speed of rotation of the crankshaft. The secondary harmonic results in a smaller shaking force having a frequency equal to twice the speed of the crankshaft.
In a multicylinder engine, the shaking forces produced by the reciprocating parts are balanced as much as possible by causing the shaking forces from the separate cylinders to oppose each other, usually in the form of opposing couples. This is easier to accomplish with a larger number of cylinders, because more cylinders give more possibilities for obtaining a favorable arrangement of cranks. Eight cylinders have been found to afford several different acceptable engine configurations, all having complete primary and secondary balance. An example is the ninety-degree, V-8 engine. This type has been widely adopted in the larger American automobiles, because in addition to having good balance, it is a very compact engine.
Six cylinders afford fewer configurations having the desired balance for automotive use than eight cylinders. A common type of six-cylinder engine is the in-line six. This engine has acceptable balance, but it is undesirably long for engine compartments. The V-6 engine, on the other hand, is more compact but it does not have complete primary and secondary balance without the use of an auxiliary balancing device.
Four cylinders has long been considered an ideal number for an economical engine for a small automobile. However, when the number of cylinders is reduced to four, it is no longer possible to obtain any configuration of four-cycle engine having complete primary and secondary balance without the use of an auxiliary balancing device. The most common type of four-cylinder engine for automobiles is the in-line four. In this type of engine, the secondary shaking forces add to each other instead of opposing each other. The result is that this engine has a fairly large secondary shaking force in the plane of the cylinders. This shaking force can give a perceptible vibration to an automobile at moderate highway speeds.
It has long been known that the secondary shaking forces of an in-line, four-cylinder engine can be balanced by the use of auxiliary balance shafts, which rotate at twice engine speed. This type of balancing means, however, has not been adopted to any significant extent, possibly because of its expense and operational problems.
Dimensional considerations and also cost considerations would indicate that a preferred type of engine for a small automobile would be a V-4 engine. V-4 engines are indeed indicated in the prior art. The balance of these engines has been notably poor, however. Generally, these engines have a combination of unbalanced primary couples, unbalanced secondary couples, and unbalanced secondary forces. Because of the unbalance of V-4 engines, their use has generally been limited to industrial-type applications, where a greater amount of vibration can be tolerated than in automobiles.
Another undesirable characteristic of reciprocating engines is cyclic variation of output torque. Such variation is sometimes perceptible in automobiles in the form of a vibration. An important part of this torque variation is the inertia torque produced by reciprocating parts in the engine cylinder. Cyclic variation of inertia torque is worse in four-cylinder engines than it is in engines having a greater number of cylinders. This is due to the fact that the inertia torques from all cylinders add to each other in even-firing, four-cylinder engines. All conventional four-cylinder engines, therefore, have this disadvantage.